This invention is generally directed to carrier and developer compositions thereof, and more specifically, the present invention relates to developer compositions with synthetic carrier components, and yet more specifically, the present invention relates to melt mixing processes for the high solids content microsynthetic carrier wherein the solids content is in the range amount of from about 60 to about 90 percent by weight of carrier, and wherein the term xe2x80x9cmicroxe2x80x9d of xe2x80x9cmicrosyntheticxe2x80x9d refers, for example, to a synthetic carrier particle size in the range of from about 20 to about 100 microns, and more specifically, from about 25 to about 80 microns in diameter, and preferably from about 30 to about 60 microns in diameter.
In embodiments of the present invention, the synthetic carrier particles are generally comprised of binder polymer, magnetic component, colorant, and optionally charge control additives. The carriers, which can be prepared by extrusion melt mixing, pelletizing, grinding and classification, can be selected for a number of different xerographic systems, such as copiers and printers like high speed color xerographic copies, digital copiers, multi copier/printers, and more specifically, wherein colored copies with excellent and substantially no background deposits are achievable. Developer compositions comprised of the carrier particles illustrated herein are generally useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging and printing processes, and digital processes. Additionally, the invention developer compositions can be selected for a number of imaging processes such as noninteractive development systems, reference U.S. Pat. Nos. 4,292,387; 5,409,791 and 5,826,151, the disclosures of which are totally incorporated herein by reference.
The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions are the appropriate triboelectric charging values associated therewith.
Certain synthetic carriers are known, for example U.S. Pat. No. 4,426,433, which discloses a carrier with a binder and a powder of a magnetizable material dispersed therein, and carbon black. The resin binder includes styrene butadiene polymers, and the magnetite can be MAPICO BLACK(trademark). Also, U.S. Pat. No. 5,663,027 discloses a carrier of a binder resin, such as a polyester, or a styrene/acrylic copolymer, and a magnetite such as FeO.Fe2O3. In U.S. Pat. No. 4,565,765, there is illustrated a carrier composition comprised of a resin binder of for example, polyamides, epoxies, polyurethanes, polyesters, styrene acrylates, and magnetites like MAPICO BLACKS(trademark) Carbon black can also be included in the carrier according to the disclosure of U.S. Pat. No. 4,565,765. Moreover, in U.S. Pat. No. 5,629,119 there is disclosed melt kneading processes for the preparation of a two component binder type magnetic carrier comprised of a magnetic powder and a binder resin wherein the carrier selected contains dispersed therein a release agent.
Nonsynthetic carriers are disclosed, for example, in U.S. Pat. No. 3,590,000. These carrier particles can contain various cores, including steel, with a coating thereover of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds. A number of these coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where part of, or the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, usually adversely effect the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity, and relatively low tribo as compared to the high tribo carriers of the present invention.
There are illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic or thermosetting resin particles. The resulting mixture is then dry blended until the resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320xc2x0 F. to about 650xc2x0 F. for a period of 20 minutes to about 120 minutes, enabling the resin particles to melt and fuse on the carrier core.
There is illustrated in U.S. Pat. Nos. 4,937,166 and 4,935,326, the disclosures of which are totally incorporated herein by reference, carrier containing a mixture of polymers, such as two polymers, not in close proximity in the triboelectric series. Moreover, in U.S. Pat. No. 4,810,611, the disclosure of which is totally incorporated herein by reference, there is disclosed the addition to carrier coatings of colorless conductive metal halides in an amount of from about 25 to about 75 weight percent, such halides including copper iodide, copper fluoride, and mixtures thereof.
There are disclosed in U.S. Pat. No. 4,565,765 processes for the preparation of synthetic carriers containing a MAPICO BLACK(trademark) magnetite up to 60 percent by weight of carrier, and VULCAN XC72R(trademark) carbon black up to 8 percent by weight of carrier. The compositions can be ground in a Fitzmill and screened to an average particle size of about 75 microns. The MAPICO BLACK(trademark) magnetite disclosed in U.S. Pat. No. 4,565,765 has a coercivity less than 200 gauss, and therefore is considered soft magnetic. To prepare a hard magnetic carrier, there is selected a hard magnetic powder such as, for example, strontium ferrite which is more insulative than MAPICO BLACK(trademark) magnetite. The induced magnetic moment of a synthetic carrier in an applied magnetic field is a function of the concentration of magnetic material in the carrier particle. It is, therefore, preferred to maximize the amount of magnetic material contained in the carrier particle. To minimize the effect of magnetic moment reduction by dilution of magnetic material in the carrier particle by the binder resin and carbon black, it is desirable to have a concentration of magnetic material greater than 50 percent by weight, that is, in the range from about 50 to about 85 percent by weight, and preferably in the range from about 60 to about 75 percent by weight of carrier.
Other U.S. patents that may be of interest include U.S. Pat. No. 3,939,086, which illustrates steel carrier beads with polyethylene coatings, see column U.S. Pat. No. 6; 4,264,697, which discloses dry coating and fusing processes; U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935; and 4,434,220, the disclosures of each of these patents being totally incorporated herein by reference.
For specified interactive or noninteractive development systems, such as TurboMAZE, it is desirable for the carrier in the developer to possess hard magnetic characteristics. By hard magnetic is meant, for example, that the carrier has a magnetic coercivity greater than about 200 gauss, for example from about 300 to about 6,000 gauss as measured with a vibrating sample magnetometer. Hard magnetic refers, for example, generally to a material with permanent magnetic characteristics, that is the carrier possesses magnetic characteristics in the presence and in the absence of a magnet, and possesses a coercivity greater than about 200 gauss. Soft magnetic refers, for example, to a material that possesses magnetic characteristics in the present of a magnet and does not possess magnetic characteristics in the absence of a magnet.
In situations where a conductive carrier is preferred, it is desirable to have a conductive binder resin, that is, wherein the binder resin contains sufficient amounts of a conductive additive such as, for example, conductive carbon black to render the carrier particle conductive. By conductive, it is meant, for example, that the carrier has a conductivity greater than about 10xe2x88x929 mho/cm. In U.S. Pat. No. 4,565,765, conductive carbon black concentrations of about 8 percent by weight of carrier are disclosed. Together with the MAPICO BLACK(trademark) magnetite, this level of carbon black renders the carrier particle conductive. Strontium ferrite is more insulative than MAPICO BLACK(trademark). Therefore, to attain similar levels of conductivity as in U.S. Pat. No. 4,565,765, significantly higher amounts of conductive carbon black need to be utilized, such as for example, in the range from about 10 to about 20 per cent by weight of carrier. Important to the conductivity is also how the carbon black is dispersed in the binder resin. If the carbon black is dispersed too finely, then the carrier conductivity will be lower than if the level of dispersion is more moderate. Processing conditions, for example melt mixing process, are needed to optimize the carbon black dispersion for maximized conductivity.
In addition to imparting conductivity to the carrier, the presence of carbon black in synthetic carrier particles can function to improve the adhesion between the binder resin and magnetic component. This is especially important for concentrations of magnetic component in excess of about 60 percent by weight of carrier. For the sole purpose of improving adhesion between the binder resin and magnetic powder, amounts of carbon black selected are in the range of from about 1 to about 20 percent by weight of carrier. When no carbon black is present, the carrier containing a high loading of magnetic component is very fragile in a developer housing. Furthermore, in the process of grinding of the synthetic carrier, excess amounts of fines would be produced, that is in excess of 50 percent by weight of material with particle size less than about 25 microns, rendering the process uneconomical.
For an insulative synthetic carrier, the desirable concentration of carbon black is in the range of from about 1 to about 20 percent by weight of carrier. The desirable concentration of carbon black for a conductive synthetic carrier is in the range of from about 10 to about 20 percent by weight of carrier. Therefore, the solids content (magnetic component and carbon black) of a hard magnetic synthetic carrier can range from about 60 to about 90 percent by weight of carrier, and wherein a very low amount of binder resin remains in the carrier to bind the carrier together, that is, for example from about 10 to about 40 percent by weight of carrier. At these low levels of binder resin, special precautions are needed to prevent undue wear in polymer melt mixing machinery, such as for example, downstream feeding of the magnetic powder in an extruder.
It is known that the addition of carbon black to a binder resin has the effect to reinforce the binder resin, that is, to increases the tensile strength, toughness and hardness of binder resin in addition to improving the adhesion between the binder resin and magnetic powder. For carbon black loading in excess of about 10 percent by weight, this increase is substantial which results in a decrease in the brittleness or grindability of the binder resin thereby render the processes for the preparation of small carriers difficult and uneconomical. In specified interactive or noninteractive development systems, such as TurboMAZE, smaller carrier particles are desired, such as for example those with a volume average diameter of from about 25 to about 100 microns, and preferably from about 30 to about 60 microns as measured with a Coulter Counter.
To generate smaller carrier particles with higher carbon black contents, a new grinding process is needed since fitzmilling alone will not suffice. Therefore, there is needed a process for the generation of high solids content, small size microsynthetic carriers where the process includes (i) melt mixing to enable the processing of high concentrations of solids (magnetic powder and carbon black), and (ii) grinding and classification for the generation of small size microsynthetic carrier particles in the size range of from about 25 and 100 microns, and preferably from about 30 to about 60 microns.